StopWatch.cs

namespace StopWatch
{
    public class StopWatch
    {
        private DateTime StartTime { get; set; }

        public void Start() => this.StartTime = DateTime.Now;

        public TimeSpan Stop() => DateTime.Now.Subtract(StartTime);
    }
    
    internal class Program
    {
        private static void Main(string[] args)
        {
            ConsoleKey userChoice;
            do
            {
                Console.WriteLine("\n\nPress the 'Space' key to Start timer, any other button to exit.");
                userChoice = Console.ReadKey().Key;
                if(userChoice != ConsoleKey.Spacebar)break;

                var watch = new StopWatch();
                watch.Start();
                Console.WriteLine("\nTimer Started.. press Space to Stop timer, any other button to exit.");

                userChoice = Console.ReadKey().Key;
                var duration = watch.Stop();
                Console.WriteLine($"\nDuration: {duration.Hours}:{duration.Minutes}:{duration.Seconds}");
            } while (userChoice == ConsoleKey.Spacebar);

            Console.WriteLine("\nExiting timer....");
        }
    }
}